Coating system for superalloys

ABSTRACT

A protective coating system is provided for nickel-base and cobalt-base superalloys which is capable of imparting oxidation and corrosion resistance at elevated temperatures. The superalloy body is first coated by physical vapor deposition with a composition consisting essentially of chromium, aluminum, a member selected from the group consisting of yttrium and the rare earth elements, and at least one element selected from the group consisting of iron, cobalt and nickel, and thereafter the body is subjected to an aluminizing overcoating to increase the corrosion resistance.

1451 Mar. 25, 1975 COATING SYSTEM FOR SUPERALLOYS Inventors: James L.Walker; John R. Ross,

both of Schenectady, N.Y.

Assignee: General Electric Company,

Schenectady, NY.

Filed: Apr. 2, 1973 Appl. No.: 346,919

11.8. Cl. 117/71 M, l17/107.2 P Int. Cl. B44d l/16 Field of Searchll7/7l M, 107, 107.2 P

References Cited UNITED STATES PATENTS 7/1971 Maxwell et al 117/107.2 P2/1972 Schwartz et a1. 117/107.2 P 7/1972 Evans ct al. 117/107 PrimaryExaminer-Cameron K. Weiffenbach Attorney, Agent, or Firm--Gerhard K.Adam; Joseph T. Cohen; Jerome C. Squillaro [5 7] ABSTRACT A protectivecoating system is provided for nickelbase and cobalt-base superalloyswhich is capable of imparting oxidation and corrosion resistance at ele'vated temperatures. The superalloy body is first coated by physicalvapor deposition with a composition consisting essentially of chromium,aluminum, a member selected from the group consisting of yttrium and therare earth elements, and at least one element selected from the groupconsisting of iron, cobalt and nickel, and thereafter the body issubjected to an aluminizing overcoating to increase the corrosionresistance.

8 Claims, 5 Drawing Figures COATING SYSTEM FOR SUPERALLOYS Thesuperalloys are heat-resistant materials having superior strengths athigh temperatures. Many of these alloys contain iron, nickel or cobaltalone or in combination as the principal alloying elements together withchromium to impart surface stability and usually contain one or moreminor constituents, such as molybdenum, tungsten, columbium, titaniumand aluminum for the purpose of effecting strengthening. The physicalproperties of the superalloys make them particularly useful in themanufacture of gas turbine engine components.

Heretofore, surface coatings have been used to protect the superalloyarticles from high temperature oxidation and corrosion. Various coatingsfor superalloys have been described in the literature and of particularinterest are coating compositions consisting essentially of chromium,aluminum, a member selected from the group consisting of yttrium and therare earth elements and a metal selected from the group consisting ofiron, cobalt, and nickel. Illustrative coatings wherein the compositionsare given in weight percent are designated as follows:

The application of the coating composition to a variety of substrates,such as nickel-base and cobalt-base superalloys may be achieved byphysical vapor deposition in a vacuum chamber. During this procedure,the composition is thermally evaporated from a source heated, forexample, by an electron beam, and a thin metal coating is condensed onthe surface of the workpiece. Layers of the coating are formed as theworkpiece is rotated until the thickness is in the range of about 3-5mils. Unfortunately, the deposited coating has radially oriented defectswhich are the sites of attack by oxidizing and/or corrosive atmospheresat high temperatures. Such defects can lead to premature failure of thecoating.

Attempts to prolong the useful life of superalloys coated with a FeCrAlYalloy are disclosed by Elam, et al., US. Pat. No. 3,528,861. The coatingeffectiveness was found to be limited by the formation of anintergranular precipitate during the coating deposition cycle. Theeffect of the detrimental precipitate was improved by shot peening orglass bead blasting to bteak up the precipitate into small particleswhich are more easily taken into solution by heat treatment.

In accordance with the present invention, we have invented a method ofimproving the high temperature corrosion resistance of a nickel-base orcobalt-base superalloy body by first coating the superalloy body byphysical vapor deposition with a composition consisting essentially ofchromium, aluminum, a member selected from the group consisting ofyttrium and the rare earth elements, and at least one element selectedfrom the group consisting of iron, cobalt and nickel and thereafteraluminizing the coated body by chemical vapor deposition to increase thecorrosion resistance of the body. The effectiveness of the coatingsystem may be explained by the fact that the first coating exhibitsgrain boundaries that are oriented in a perpendicular direction to thedeposition plane. Upon exposure to a corrosive environment, theseboundaries are preferentially attacked resulting in ultimate failure ofthe coating. The application of an aluminizing overcoat prevents thistype of failure and thereby substantially increases the life of thecoated article. In addition, the concentration profile of our novelcoating system indicates the presence of a high concentration ofaluminum on the outer surface of the coating which may also contributeto the improved properties. The coated superalloy bodies prepared by ourinvention are particularly useful in making gas turbine enginecomponents.

The invention is more clearly understood from the following descriptiontaken in conjunction with the accompanying drawing in which:

FIG. 1 is a photomicrograph 500x of a Rene 8O nickel-base superalloybody coated with a Ni- CrAlY coating.

FIG. 2 is a photomicrograph (500 of a Rene nickel-base superalloy bodycoated with a first Ni- CrAlY coating and then treated with analuminizing overcoat according to the method of our invention.

FIG. 3 is a photomicrograph (SOOX) illustrating the effect of corrosionon a CoCrAlY coated superalloy body.

FIG. 4 is a photomicrograph (500 illustrating the effect of corrosion ona superalloy body coated first with a CoCrAlY coating and then treatedwith an aluminizing overcoat.

FIG. 5 is a microprobe profile of a body prepared by our invention andshowing the high surface gradient of aluminum.

The superalloys are strong, high temperature materials which areparticularly useful in gas turbine engines. A substantial listing ofthese materials is set forth by W. F. Simmons, Compilation of ChemicalCompositions and Rupture Strengths 0f Superalloys, ASTM Data SeriesPublication No. DS9E, and may be represented by the nominal compositionsin weight percent of the following superalloys:

The first coating of our protective coating system is designated hereinas MCrAlY" coating wherein M is a member selected from the groupconsisting of iron, cobalt, and nickel. This coating is broadly definedas consisting essentially in weight percent of the following nominalcompositions:

Ingredients Weight Chromium 14 -35 Aluminum 4 20 Yttrium 0.1- 3 IronCobalt Balance Nickel Included in this formulation are the compositionsdesignated hereinabove as FeCrAlY, CoCrAlY, and Ni- CrAlY. The MCrAlYcoating is applied to the substrate by a physical vapor depositiontechnique which is described in considerable detail in Vapor Deposition,Edited by C. F. Powell, et al., John Wiley & Sons, New York (1966).Accordingly, the coating is evaporated and deposited in a vacuumchamber. Typically, the metal alloy is heated by an electron beamfocused on the metal alloy ingot to evaporate the metal to a vapor.During evaporation, the vapor condenses as a coating, preferably about3-5 mils in thickness on the workpiece being coated. The material to beapplied is heated in a high vacuum to a temperature at which its vaporpressure is about l"torr or greater whereupon it emits molecular rays inall directions. During coating the vacuum must be very high to permitthe molecular rays to travel from their source without disturbance untilthey hit the surface of the object to be coated. A photomicrograph of anickel-base superalloy coated with a NiCrAlY coating is shown in FIG. 1.

Thereafter, the first coating is treated by an aluminizing overcoat by achemical vapor deposition technique such as illustrated by Levine, etal., U.S. Pat. No. 3,540,878, and as discussed in Powell, et al., citedhereinabove. In a preferred embodiment, the aluminizing is performed bya pack-cementation method in which the article is packed in a porousmixture of refractory particles and granular aluminum or an aluminumcontaining alloy and heated to between 600-l ,000 C. in the presence ofa halide salt activator. In a specific embodiment as disclosed byLevine, et al., the particulate pack mixture includes a powder of amultiphase ternary alloy of Ti, Al and C, an inert filler which will notreact with the other components of the mixture to prevent powdersintering, and a halide salt activator such as member selected from thechlorides and fluorides of ammonia and the alkali 'metals. The mostpractical activator is a halide salt selected from NaF, KF, NH C1 andNI-I F in an amount of about 01-10 percent by weight of the mixture. Thepreferred filler material is refractory alumina powder which comprisesabout -985 weight percent of the total pack powder. During preparationof such a mixture, the filler powder, the powdered ternary alloy and theactivator are blended together in a conventional mixing apparatus suchas an ordinary powder blender. An illustrative pack contains about 4percent by weight of the ternary alloy of Ti, Al and C. Aphotomicrograph of a nickel-base superalloy which has been coated with afirst coating of a NiCr- AIY alloy and then an aluminizing overcoatingis shown in FIG. 2. The first coating exhibited grain boundaries thatare oriented in a perpendicular direction to the deposition plane, whichbecome sites for attack by high temperature oxidation and corrosion.Upon application of the aluminizing overcoat, any open defects of anMCrAlY coating become filled and a high concentration of aluminum isdeposited on the outer surface of the coating as shown in FIG. 5 (aconcentration profile of a CoCrAlY coated Rene 80 body with analuminizing overcoating). It is to this unique coating system that weattribute the improved properties of high temperature oxidation andcorrosion resistance.

Our invention is further illustrated by the following examples.

EXAMPLE I Ingredient Weight Cobalt 64 Chromium 22 Aluminum l 3 Yttrium IEvaporation of the metal was at a constant power of 19.0 kilovolts and275 milliamps for 30 minutes. The pins were then cooled in vacuum. Acoating having a thickness of about 3 mils was deposited on the pins.

One group of coated pins was then lightly sand blasted to prepare thesurface for vapor deposition. The aluminizing mixture was prepared bymixing 30 g. of+200-350 mesh NiAl powder and 270 g. of alumina in asuitable container. Then 300 ml. ofa 0.2% aqueous NH F solution wasadded and the contents heated to about 300 C., while mixing occasionallyto remove the water by evaporation. The completely dry powder was putinto an Inconel metal box (with two holes in the top of each end) in anamount of at least 3 g. of powder to each square centimeter of surfaceto be aluminized. The CoCrAlY coated samples were put in the box andcompletely covered by the powder. The box was then covered and placed ina retort that had approximately 0.5 cu. ft./hr. of hydrogen flowingthrough it and placed in a furnace. The furnace was heated to 850 C.,held at that temperature for 1 hour and then cooled to room temperature.Upon examination of the sample it was noted that this procedure resultedin a penetration of about l-2 mils of aluminum into the surface of theCoCrAlY coating.

A crucible test was then performed to test resistance to oxidation andcorrosion of the samples coated only with the CoCrAlY coating and thesecond group subjected to a subsequent aluminizing procedure. Bothgroups of coated pins were immersed in a bath of Na SO V O (%:25% byweight) at a temperature of 900 C., while gaseous oxygen was bubbledthrough the bath. After 18 hours the samples were removed.

It was observed that the samples coated only with the CoCrAlY coatingwere characterized by deep spike" corrosion, but the pins subjected tothe aluminizing overcoating procedure were substantially more resistantto attack by corrosion.

EXAMPLE II Following the procedure of Example I, test pins having adiameter of one-eighth in. were prepared from Rene 80 nickel-basesuperalloy. The pins were coated by electron beam evaporation with thefollowing nominal composition:

After the pins were removed from the apparatus, the coating had athickness of 3-5 mils.

Thereafter the coated pins were aluminized by the pack cementationtechnique. A pack composition was prepared to meet the followingspecification in weight percent: 60% Ti, 33.5% Al, and 4.8-5.6% C. A 1V2 percent by weight pack was prepared by diluting the pack in 98.5% byweight of A1 The coated pins were then embedded in the powder mixtureand aluminized at a temperature of l,925 F. for 4 hours.

Comparative high temperature oxidation and corrosion tests wereperformed on test samples coated only with the CoCrAlY alloy and on testsamples which had been subsequently subjected to the aluminizingovercoat. In the crucible test, the pins were partially immersed insodium sulfate and a mixture of 80 parts by weight of sodium sulfate and20 parts by weight of vanadium pentoxide for a time of 16 hours. Thesamples coated only with the CoCrAlY coating after being subjected to atemperature of l,925 F. for 1 hour in vacuum are shown in FIG. 3.Typical spike corrosion and penetration were observed. The samples whichhad been protected by the aluminizing overcoat, after being subjected toa temperature of 1,925 F. for 4 hours in vacuum are shown in FIG. 4. Theresults indicated almost a complete absence of spike corrosion and thatthe aluminized coating had filled in the defects of the initial coating.

Another group of samples was subjected to a burner rig test whichsimulated conditions used in a gas turbine engine. The test was run of2,000 hours at a temperature of l,475 F. using a jet fuel containing 1ppm. sea salt and 5 ppm. of vanadium pentoxide. The results indicatedthat CoCrAlY coated samples were heavily attacked whereas the aluminizedovercoated samples were still intact.

EXAMPLE III Following the procedure of Example I, cast pins of Rene 80nickel-base superalloy were coated by vapor deposition with thefollowing nominal composition:

b Ingredient Weight Nickel 6 l .5 Cobalt 9.5 Chromium 25.0 Aluminum 3.0Yttrium 1.0

A coating having a thickness of about 3 mils was deposited on the pins.Some of the pins were then aluminized by the pack cementation techniquedescribed in Example I.

When subjected to the corrosion tests it was observed that the pinssubjected to the aluminizing overcoat procedure were considerably moreresistant to corrosion than those which had been coated only with theMCrAlY coating.

It will be appreciated that the invention is not limited to the specificdetails ssown in the examples and illustrations and that variousmodifications may be made within the ordinary skill in the art withoutdeparting from the spirit and scope of the invention.

We claim: I

l. A method of improving the high temperature oxidation and corrosionresistance of a nickel-base or a cobalt-base superalloy body comprisingthe steps of:

a. coating the superalloy body by physical vapor deposition with acomposition consisting essentially of chromium, aluminum, a memberselected from the group consisting of yttrium and the rare earthelements, and at least one element selected from the group consisting ofiron, cobalt and nickel, and b. subjecting the coated body to analuminizing overcoating by pack cementation to increase the oxidationand corrosion resistance of the coating.

2. The method of claim 1, wherein said composition consists essentiallyin weight percent of 14-35% chromium, 4-20% aluminum, 0.l-3% yttrium andthe balance being a member selected from the group consisting of iron,cobalt, nickel, and mixtures thereof.

3. The method of claim 1, wherein said composition consists essentiallyin weight percent of 25-29% chromium, 12-14% aluminum, 06-09% yttriumand the balance being iron.

4. The method of claim ll, wherein said composition consists essentiallyin weight percent of 19-24% chromium, 13-17% aluminum, 06-09% yttrium,and the balance being cobalt.

5. The method of claim 1, wherein said composition consists essentiallyin weight percent of 20-35% chromium, 15-20% aluminum, 0.050.30% yttriumand the balance being nickel.

6. The method of claim 1, wherein said body is a nickel-base superalloy.

7. The method of claim 1, wherein said body is a cobalt-base superalloy.

8. The method of claim 1, wherein the first coating has a thickness ofabout 3-5 mils and the aluminizing overcoating penetrates into the firstcoating to a depth of about l-2 mils.

1. A METHOD OF IMPROVING THE HIGH TEMPERATURE OXIDATION AND CORROSIONRESISTANCE OF A NICKEL-BASE OR A COBALT-BASE SUPERALLOY BODY COMPRISINGTHE STEPS OF: A. COATING THE SUPERALLOY BODY BY PHYSICAL VAPORDEPOSITION WITH A COMPOSITION CONSISTING ESSENTIALLY OF CHROMIUM,ALUMINUM, A MEMBER SELECTED FROM THE GROUP CONSISTING OF YTTRIUM AND THERARE EARTH ELEMENTS, AND AT LEAST ONE ELEMENT SELECTED FROM THE GROUPCONSISTING OF IRON, COBALT AND NICKEL, AND B. SUBJECTING THE COATED BODYTO AN ALUMINIZING OVERCOATING BY PACK CEMENTATION TO INCREASE THEOXIDATION AND CORROSION RESISTANCE OF THE COATING.
 2. The method ofclaim 1, wherein said composition consists essentially in weight percentof 14-35% chromium, 4-20% aluminum, 0.1-3% yttrium and the balance beinga member selected from the group consisting of iron, cobalt, nickel, andmixtures thereof.
 3. The method of claim 1, wherein said compositionconsists essentially in weight percent of 25-29% chromium, 12-14%aluminum, 0.6-0.9% yttrium and the balance being iron.
 4. The method ofclaim 1, wherein said composition consists essentially in weight percentof 19-24% chromium, 13-17% aluminum, 0.6-0.9% yttrium, and the balancebeing cobalt.
 5. The method of claim 1, wherein said compositionconsists essentially in weight percent of 20-35% chromium, 15-20%aluminum, 0.05-0.30% yttrium and the balance being nickel.
 6. The methodof claim 1, wherein said body is a nickel-base superalloy.
 7. The methodof claim 1, wherein said body is a cobalt-base superalloy.
 8. The methodof claim 1, wherein the first coating has a thickness of about 3-5 milsand the aluminizing overcoating penetrates into the first coating to adepth of about 1-2 mils.